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Reserve for Unpaid Losses and Loss Adjustment Expenses - Additional Information (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2016
Dec. 31, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]          
Unpaid losses and loss adjustment expenses $ 42,318 $ 41,243      
Asbestos and Environmental          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Reinsurance, Excess Retention, Amount Reinsured, Per Policy 62        
Reinsurance, Amount Retained, Per Policy       $ 1,700  
Change in Deferred Gain on Retroactive Reinsurance 194 229 $ 155    
Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Change in Deferred Gain on Retroactive Reinsurance 0 0 91    
Reinsurance Policy, Type [Axis]: Asbestos and Environmental | Asbestos and Environmental          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Reinsurance, Amount Retained, Per Policy       $ 1,700  
Retention Layer for Reserve for the Covered Liabilities as of the Inception Date [Member] | Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Reinsurance, Amount Retained, Per Policy 1,816        
Retention Layer Above Reserve for the Covered Liabilities as of the Inception Date [Member] | Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Reinsurance, Amount Retained, Per Policy 100        
Group Benefits          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Prior accident year development [1] (502) (381) (458)    
Unpaid losses and loss adjustment expenses 8,274 8,160 8,210   $ 8,233
Group Benefits | Group long-term disability          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Unpaid losses and loss adjustment expenses 6,863        
Group Benefits | Group disability          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Prior accident year development [1] 457 325 380    
Group Benefits | Group life and accident, excluding premium waiver          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Prior accident year development [1] 36 50 65    
Unpaid losses and loss adjustment expenses 744        
Group Benefits | Group supplemental health          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Prior accident year development [1] 9 $ 6 $ 10    
Unpaid losses and loss adjustment expenses $ 38